1. Field of the Invention
The present invention relates to a grain-oriented magnetic steel sheet having a low iron loss, suitable for use as an iron core material mainly for electric power transformers and rotary machines.
2. Description of the Related Art
When manufacturing a grain-oriented magnetic steel sheet, it is a common practice to use a precipitate known as an inhibitor to produce secondary recrystallized grains of a Goss orientation ({110} less than 001 greater than ) during final finish annealing.
Representative methods so far disclosed include method using AlN and MnS as disclosed in Japanese Patent Publication No. 40-15644 and a method using MnS and MnSe as disclosed in Japanese Patent Publication No. 51-13469, both having already been industrialized.
Apart from the above, adding CuSe and BN as disclosed in Japanese Patent Publication No. 58-42244, using nitrides such as those of Ti, Zr and V as disclosed in Japanese Patent Publication No. 46-40855; many other methods are known.
These methods using inhibitors are useful for stably producing secondary recrystallized grains. However, because precipitates must be finely dispersed, it is necessary that the slab heating temperature before hot rolling is at least 1,300xc2x0 C. Heating of the slab to a high temperature requires higher equipment cost, and in addition, results in an increase in the quantity of scale produced during hot rolling. This leads to many problems such as a lower product yield and a more complicated equipment maintenance.
Another problem involved in the methods using inhibitors is that these inhibitor constituents, if remaining after the final finish annealing, cause deterioration of the magnetic properties. For the purpose of eliminating these inhibitor constituents Al, N, B, Se and S, therefore, purification annealing is carried out for several hours in a hydrogen atmosphere at a temperature of at least 1,100xc2x0 C. after completion of secondary recrystallization. However, purification annealing carried out at such a high temperature leads to problems of a lower mechanical strength of the steel sheet, bucking of the lower part of the coil, and a considerably lower product yield.
It is true that, as a result of this high-temperature purification annealing, the contents of Al, N, B, Se and S in steel are reduced to up to 50 ppm, respectively. These constituents are however concentrated in the forsterite film; in the interface between the film and the iron substrate these constituents remain inevitably as single substances or as compounds. Presence of these substances prevents movement of a magnetic domain wall and causes an increase in iron loss. Further, these substances present in the film/iron interface inhibit grain boundary displacement of the crystal grains directly under the film. As a result, fine grains not completely encroached by secondary recrystallized grains are often present directly below the surface layer. Presence of such fine grains also causes deterioration of the magnetic properties. Moreover, it is still difficult to eliminate Nb, Ti and V even by high-temperature purification annealing, and this is also a cause of deterioration of iron loss.
The manufacturing methods of a grain-oriented magnetic steel sheet using inhibitors faces the problem of a high cost as described above; achievement of a lower iron loss is also limited. In order to avoid these problems, we have considered use of a method not using an inhibitor.
There are known manufacturing methods of a grain-oriented magnetic steel sheet without using an inhibitor such as those disclosed in Japanese Unexamined Patent Publication No. 64-55339, No. 2-57635, No. 7-76732 and No. 7-197126. One of features common to these techniques is that it is intended to preferentially cause growth of grains having the {110} orientation, using the surface energy as a driving force.
In order to effectively utilize the difference in surface energy, it is necessarily required to use a thin sheet for increasing the contribution of the surface. For example, the technique disclosed in Japanese Unexamined Patent Publication No. 64-55339 limits the thickness to up to 0.2 mm, and the technique disclosed in Japanese Unexamined Patent Publication No. 2-57635, to up to 0.15 mm. The technique disclosed in Japanese Unexamined Patent Publication No. 7-76732, not particularly limiting the thickness, reveals a very poor orientational integration as typically represented by a magnetic flux density of up to 1.700 T for B8 for a thickness of 0.30 mm according to Example 1 presented in the specification thereof. In the examples shown therein, the thickness giving a satisfactory magnetic flux density is limited to 0.10 mm. In a technique disclosed in Japanese Unexamined Patent Publication No. 7-197126 also, the thickness is not limited, but the technique is for applying a tertiary cold rolling of from 50 to 75%. The thickness necessarily becomes smaller: a thickness of 0.10 mm is proposed in an example shown in the Publication.
Most of the thicknesses of grain-oriented magnetic steel sheet now commonly use at least 0.20 mm. That is, it is difficult to obtain a product generally in use by a method using the surface energy as described above.
Further, in order to utilize surface energy, it is necessary to carry out the final finish annealing at a high temperature inca state in which the growth of surface oxides is inhibited. For example, Japanese Unexamined Patent Publication No. 64-55339 discloses a technique of using vacuum, an inert gas, or a mixed gas of hydrogen and nitrogen as an annealing atmosphere at a temperature of at least 1,180xc2x0 C. Japanese Unexamined Patent Publication No. 2-57635 recommends using an inert gas, hydrogen or a mixed gas of hydrogen and an inert gas as an annealing atmosphere at a temperature of from 950 to 1,100xc2x0 C., and further, reducing the pressure of the atmospheric gases. Japanese Unexamined Patent Publication No. 7-197126 discloses a technique of carrying out final finish annealing at a temperature within a range of from 1,000 to 1,300xc2x0 C. in a non-oxidizing atmosphere having an oxygen partial pressure of up to 0.5 Pa or in vacuum.
When desiring to obtain satisfactory magnetic properties by the use of surface energy, as described above, the atmosphere for the final finish annealing must be an inert gas or hydrogen, and a vacuum is suggested as a recommended condition. However, it is very difficult to use a high temperature and a vacuum simultaneously in equipment, further leading to high cost.
When utilizing surface energy, only the grains having the {110} plane are selected to grow. In other words, unlike secondary recrystallization using an inhibitor, it is not always possible that a Goss grain growth with the  less than 001 greater than  orientation aligned with the rolling direction is selected. Magnetic properties of a grain-oriented electromagnetic steel sheet are improved only when the easy axis of magnetization  less than 001 greater than  is aligned to the rolling direction. Satisfactory magnetic properties are unavailable in principle with the selection of grains having the {110} plane alone. That is, satisfactory magnetic properties are available only under very limited rolling conditions or annealing conditions in a method using surface energy. As a result, magnetic properties of a steel sheet available by use of surface energy are inevitably very unstable.
In a method using surface energy, furthermore, formation of a surface oxide layer must be inhibited during final finish annealing. In other words, an annealing separator such as MgO cannot be coated for annealing. It is therefore impossible to form an oxide film similar to that of an ordinary grain-oriented magnetic steel sheet manufactured using an inhibitor after final finish annealing. For example, a forsterite film is an oxide film formed on an ordinary grain-oriented magnetic steel sheet surface made by using an inhibitor upon coating an annealing separator mainly comprising MgO. The forsterite film not only imparts a tension to the steel sheet surface, but also exerts a function of ensuring adhesion of an insulating tensile coating mainly comprising a phosphate to be coated and baked. In the absence of a forsterite film, therefore, there is a large deterioration in iron loss.
More specifically, the use of surface energy, known as a manufacturing technique of a grain-oriented magnetic steel sheet not using an inhibitor, encounters problems of a limited thickness of steel sheet, a poor accumulation of secondary recrystallized grain orientations, and a deterioration of iron loss caused by the absence of a surface oxide film.
The present invention provides a manufacturing method not using an inhibitor, which permits avoidance of the problems encountered when using an inhibitor, resulting from the high-temperature slab heating before hot rolling and the high-temperature purification annealing after secondary recrystallization. The invention has an object to provide a favorable solution of the problems necessarily resulting from non-use of an inhibitor but using surface energy, including limited range of steel sheet thickness, poor accumulation of the secondary recrystallized grain orientation, and deterioration of iron loss caused by the absence of a surface oxide film.
More particularly, an object of the invention is to create a grain-oriented magnetic steel sheet which, even when an inhibitor is not used, does not limit the steel sheet thickness, is free from deterioration of the accumulation of secondary recrystallized grain orientation, and permits effective improvement of iron loss through positive formation of a surface oxide film.
This invention proposes also creation of a secondary recrystallized grain texture and secondary recrystallization annealing conditions which permit achievement of the aforementioned object.
The proposed secondary recrystallized grain texture comprises extra-fine crystal grains produced in coarse secondary recrystallized grains, and the proposed secondary recrystallization annealing conditions are materialized by the use of a temperature gradient.
More specifically, the invention provides a manufacturing method of a grain-oriented magnetic steel sheet, comprising the steps of hot-rolling a steel slab containing up to about 0.12 wt % C, from about 1.0 to 8.0 wt % Si, and from 0.005 to 3.0 wt % Mn, applying annealing to the resultant hot-rolled steel sheet as required, subjecting the resultant annealed sheet to one or more runs of cold rolling including intermediate annealing into a final thickness, then applying decarburization annealing as required, coating an annealing separator as required, and then applying final finish annealing; wherein:
(1) the O content of the steel slab is limited to up to about 30 ppm;
(2) for the entire steel sheet including an oxide film present before final finish annealing, from among impurities, the Al content is limited to up to about 100 ppm, and the contents of B, V, Nb, Se, S, and N are limited to up to about 50 ppm, and (3) during final finish annealing, the N content in the steel at least in a temperature region of from about 850 to 950xc2x0 C. is limited within a range of from about 6 to 80 ppm.
Further, the invention provides a method of manufacturing a grain-oriented magnetic steel sheet, wherein:
the N content in the steel is controlled during final finish annealing by one or more of:
(a) increasing the nitrogen partial pressure in the atmosphere at least in the temperature region of from 850 to 950xc2x0 C. during final finish annealing; or
(b) adding a nitrification accelerating agent to the annealing separator.
The invention provides also a grain-oriented magnetic steel sheet having a low iron loss having a composition containing from about 1.0 to 8.0 wt % Si, and an oxide film mainly comprising forsterite (Mg2SiO4), wherein the contents of Al, B, Se and S in the entire steel sheet including the oxide film are up to about 50 ppm, respectively.